The words “melt-forming” and “embossing” should not be used interchangeably. While a stabilizing carrier can be used with either process, traditional embossing and melt-forming are different processes. “Embossing” is essentially a deformation and relaxation process where a pattern is transferred to a material through the application of pressure and, optionally, heat. “Melt-forming” refers to selectively causing the interface between a material and a patterned stamper to behave as a relatively low viscosity fluid. Melt-forming is essentially a wetting process. The transition into the melt-forming regime is typically very abrupt and constitutes a fundamentally different phenomenon compared to traditional embossing.
In our prior U.S. Patent Application Publication Numbers: 20050173071; 20050167890; 20050167885; 20050167866; 20050160893; 20050082698; 20040150135 and 20030006535 (the disclosures of which are hereby incorporated by reference), “melt-forming” allowed achieved full replication fidelity while simultaneously minimizing thermal penetration depth. This technique allowed replication directly onto DVD thickness film (600 microns) without warp.
However, this roll-to-roll process is not suitable for directly replicating onto thinner films, for example 75 to 125 micron films used for next generation optical disks. The thermal penetration depth was too large a percentage of the film thickness, resulting in strong curl. Additionally, the force necessary to peel the hot film from the stamper distorted the track roundness (stretched the film as it was being removed from the stamper).
For these reasons, switching to “platen embossing” for the thin film work was required. The transition to platen embossing required a different approach to producing the flat parts. Rather than attempting to limit the depth of thermal processing, balancing annealing (permanent densification of the polymer as a result of thermal processing history) versus shrinkage (reversible thermal expansion and contraction effects) forces through the full thickness of the film was required. Traditional solutions to this problem involve long heating and cooling time (i.e. tens of seconds to tens of minutes. The desired goal of the present inventors was 3 seconds.
Initial work centered on melt-forming the information bearing surface of the film by using the thermal capacity of the stamper to provide a “pulse” of heat energy. An insulator between the stamper and heated backing platen limited the stamper temperature recovery time. This in combination with the bias heat, applied from the non-stamper side of the film, allowed for melt-forming the information surface and then appling a slightly lower uniform heating from both sides of the film allowed for control of warp.
Unfortunately two problems remained. The still hot film could not be handled without distorting it, and track roundness (ellipticity and higher order distortions) was extremely high. Non-roundness at the outer diameter of greater than +/−100 microns and at the inner diameter of greater than +/−50 microns was typically observed. The handling problem was eliminated by laminating the process film to a stabilizing carrier. This allowed much of the cooling process to be shifted downstream from the embossing station. It also provided a stabilizing carrier for subsequent processing steps such as sputtering.
The non-roundness problem was found to originate with elastic distortions introduced into the film when it was clamped between hot surfaces. The average temperature reached during the process was not high enough to relax these stresses in the allowable pressing time of less than ten seconds. When the press opened, the unrelieved portion of the “clamping stress” relaxed and distorted the track roundness.
An initial solution to this problem was to use a “double strike” process. The process film and carrier were pressed together and laminated in a pre-embossing step. After the press opened residual clamping stress relaxed. This produced a pre-flattened and stress relieved film laminated to a stabilizing carrier. When this stack was subsequently melt-formed, remaining elastic stress was dramatically reduced. We were able to reduce outer diameter non-roundness to about +/−20 microns. This non-roundness result is adequate for DVD disks, but needs to be reduced to under 10 microns peak-to-peak for Blu-Ray disc technology. The only way to accomplish this within the processing time limits was to increase the processing temperature.
Increased process temperature created a cooling problem. If the replicated film was separated from the stamper at or above Tg, replicated feature depth was lost. The solution to this problem was to use the stamper itself as the stabilizing carrier. The replicated film would not be laminated to the surface beneath it, but to the stamper. When the press opened, the stamper and process film would be removed and allowed to cool downstream. The film would be is from the stamper after the temperature fell well below Tg. This procedure allowed for film flatness and reduce non-roundness to below ten microns peak-to-peak. Unfortunately this approach needs to handle multiple stampers which is unacceptable as a production technique. This realization led to the next stage of process evolution.
A “fixed” stamper was needed as well as a processing temperature well above Tg to relieve film stress, cooling below Tg while the replica was in physical contact with the stamper, and a total cycle time of 3 seconds or less. This is where the concept for the “rapid thermal response” tool design originated.